Some of light-emitting devices used as displays and lightings, for example use organic electroluminescence elements (organic EL elements), for example. FIG. 19 illustrates the overview of a cross-sectional structure of a light-emitting device using a general organic EL element and how light propagates.
As shown in FIG. 19, a light-emitting device 100 using an organic EL element includes a substrate 101, a reflecting electrode 102, a light-emitting layer 103, and a transparent electrode 104 which are formed in sequence above the substrate 101, and a transparent substrate 105 provided on the transparent electrode 104. When a voltage is applied between the reflecting electrode 102 and the transparent electrode 104, the light-emitting layer 103 emits light in this light-emitting device 100. At this time, for example, as shown in FIG. 19, when light is emitted at a luminous point S in the light-emitting layer 103, this light is transmitted through the transparent electrode 104 directly or after being reflected by the reflecting electrode 102, and is incident on a point P on the surface of the transparent substrate 105 at an incidence angle θ1 with respect to the plane normal to the surface of the transparent substrate 105. This light is refracted at this point P at a refraction angle θ0 and outputted to an air layer 20.
Here, in the case where the refractive index of the transparent substrate 105 is n, when the incidence angle θ1 becomes greater than a critical angle θc=sin−1 (1/n), total reflection occurs. For example, when light from the point S in the light-emitting layer 103 is incident on a point Q on the surface of the transparent substrate 105 at an incidence angle greater than or equal to the critical angle θc, this light is totally reflected at the point P and is not outputted to the air layer 20.
With reference to FIG. 20A, the following describes light extraction efficiency when the transparent layer on the light-emitting layer 103 in FIG. 19 (transparent electrode 104 and transparent substrate 105) has a multi-layer structure. FIG. 20A illustrates how light propagates on an assumption that the transparent layer in the light-emitting device shown in FIG. 19 has the multi-layer structure.
As shown in FIG. 20A, the refractive index of the light-emitting layer 103 is nk, the refractive index of the air layer 20 is n0, and the refractive indices of multiple transparent layers provided between the light-emitting layer 103 and the air layer 20 are nk−1, nk−2, . . . , n1 in the order closest to the light emitting layer 103. The propagation orientation of light emitted at the luminous point S in the light emitting layer 103 (angle between the refracting surface and the plane normal) is θk. The refraction angles on the refracting surfaces are θk−1, θk−2, . . . , θ1, θ0 in the order closest to the light-emitting layer 103. Here, the following expression (expression 1) holds based on the Snell's Law.nk sin θk=nk−1 sin θk−1= . . . =n1 sin θ1=n0 sin θ0  (Expression 1)
The following expression (expression 2) holds from this expression 1.sin θk=sin θ0×n0/nk  (Expression 2)
Expression 2 is nothing but the Snell's Law when the light emitting layer 103 is in direct contact with the air layer 20, and represents that total reflection occurs at θk≧θc=sin−1 (n0/nk) regardless of the refractive indices of the transparent layers provided between the light emitting layer 103 and the air layer 20.
Moreover, FIG. 20B schematically illustrates a range of light which can be extracted from the light emitting layer in the light-emitting device shown in FIG. 19.
As shown in FIG. 208, light which can be extracted from the light-emitting layer 103 is contained in a pair of cones 301 and 302, i.e., a pair of cones having the light emitting point S in the light-emitting layer 103 as the apex, an angle being twice the critical angle θc as the vertex angle, and z axis along the plane normal to the refracting surface as the central axis. Assuming that the light emitted from the luminous point S radiates in all directions at an equal intensity, and transmittance on the refracting surface is 100% when the incidence angle is within the critical angle θc, the light extraction efficiency η from the light emitting layer 103 is equal to the ratio of the area obtained by cutting a spherical surface 303 with the cones 301 and 302 to the surface area of the spherical surface 303 and, is given by the following expression.η=1−cos θc  (Expression 3)
It should be noted that since the transmittance within the critical angle θc is not 100%, the actual extraction efficiency η is smaller than (1−cos θc). Moreover, the total efficiency for the light-emitting device is a value obtained by multiplying the light extraction efficiency η by the light emitting efficiency of the light-emitting layer 103.
For instance, when n0=1.0 and nk=1.457 in Expression 2, the critical angle θc=sin−1(n0/nk)=43.34 degrees and the light extraction efficiency η is small at around 1−cos θc=0.273. When nk=1.70, the light extraction efficiency η decreases to around 0.191. Thus, conventional light-emitting devices can only use around 20% of emission energy due to the total reflection. This leads to decrease in luminance and increase in power consumption.
Therefore, in order to improve light emitting efficiency without degrading viewing angle characteristics, sheets having a structure in which light incident at a critical angle or greater is transmitted irrespective of emission wavelength are suggested (Patent Literature 1).
FIG. 21 illustrates the light extraction structure disclosed in Patent Literature 1. (a) in FIG. 21 illustrates patterns of the surface structure in the light extraction structure disclosed in Patent Literature 1. (b) in FIG. 21 is a partially enlarged view of the patterns illustrated in (a) in FIG. 21. (c) in FIG. 21 is a cross-sectional view taken along the line A-A in (b) in FIG. 21.
As shown in FIG. 21, the light extraction structure 400 disclosed in Patent Literature 1 is virtually divided into small square-shaped areas having a certain width w (hereinafter referred to as “boundary width”) and a certain depth d, providing no space between the small square-shaped areas, and is a recess and projection structure in which the recessed portion 401 and the projecting portion 402 are randomly allocated.
Thus, when the light extraction structure 400 is the recess and projection structure, incident light incident at a critical angle or greater can be extracted, and light can be extracted without orientation polarization by randomly allocating recessed portions and projecting portions. Thus, total luminous flux can be increased and improvement effects for luminance and a color viewing angle can be obtained. Therefore, by using sheets having the light extraction structure disclosed in Patent Literature 1, light extraction efficiency can be improved without ruing their appearances in light-emitting devices such as displays and lightings, and improvement of luminance, reduction in power consumption and extension of life of elements can be achieved.